One of the first chemical syntheses of living forms was reported about a century ago by Stephane Leduc. He showed that a spectacular plant-like growth occurs when crystals of metal salts (FeCl₃, CuSO₄, Ni(NO₃)₂, CoCl₂) are dropped into an aqueous solution of sodium silicate (water glass) [4]. Colorful plant-like extensions grow from the surface of each crystal giving rise to the so-called “chemical gardens” (Fig. 2). At that time Stephane Leduc thought he was able to create some artificial life leading to the publication of many papers about the mechanism of life [19] and [20]!

Two physicochemical processes are actualy involved in the formation of these chemical gardens.

Biogenic silica has been used by planktonic microorganisms for their protection. Diatoms provide a wondeful example of the beautiful architectures made by life. These single-cell microalgeas are surrounded by a porous silica shell (frustule) that exhibits an increadible variety of shapes. Chemists are very interested by these microorganisms that are able to make silica glasses under ambiant conditions while high temperatures are required to make industrial glasses from molten silica. Therefore the soft solution synthesis of porous silica glasses is becoming a very popular field of research. It leads to the development of the so-called sol–gel process [3]. Sol–gel silica is made via the hydrolysis and condensation of molecular precursors such as silicon alkoxides Si(OR)₄ (R CH₃, C₂H₅…) as follows:

An hydrous silica gel is formed that transforms into a silica glass upon heating. One of the main advantages of the sol–gel route is to allow the powderless processing of glasses and ceramics. It has been widely developped for the production of fibres and coatings [17].

The sol–gel route leads to the formation of porous silica gels, but their pore size cannot be accurately controlled. A real breakthrough occurred when scientists at Mobil showed that it was possible to get ordered mesoporous silica in the presence of surfactants such as cethyltrimethylammonium chloride (CTACl) [18].

Surfactant molecules are made of a long hydrophobic alkyl tail and a polar hydrophilic head. They self-assemble in polar solvents such as water to give micelles in which all polar heads are directed toward the outside aqueous medium. Mesophases are formed when surfactants are mixed with oil and water. Mesoporous silica is then obtained via a “liquid crystal templating” process. Depending on concentration, different mesophases (hexagonal, cubic, lamellar) are obtained that behave as templates for the synthesis of porous silica. Molecular precursors, alkoxides Si(OR)₄ or silicic acid Si(OH)₄ occupy the continuous water region. Condensation leads to the formation of silica walls between the surfactants assemblies. Organic surfactants are removed via calcination leaving a porous silica network. A periodic mesoporous structure can then be obtained leading to the development of mesoporous silicas which are nowadays extensively used in the field of catalysis.

Chemists are usually looking for defect free mesoporous structures. However, it is well known that topological defects, points, lines or planes can be observed during the formation of surfactant mesophases. The energy required to stabilize these defects is much smaller than in solid crystals. Large defects several micrometers in dimensions can be easily formed. The condensation of silica around these defects leads to curved shapes so that the resulting solid particles exhibit sophisticated curves that look like biogenic silica (Fig. 3) [13] and [14].

Hexagonal mesoporous silicas, MCM-41, have been extensively studied by Ozin. They are built of assemblies of rod-like 2D hexagonal array of surfactants. Topological point defects change the orientation of the director field leading to the formation of curved structures that can initiate and direct the growth of particular forms of mesoporous silica: discoidal, toroidal, disk-like, spiral, spheroidal [25]. The condensation of silica mainly quenches the topological defect and its associated director field pattern. It is possible to draw a relation between a specific type of defect and a particular mesoporous silica shape [26]. Each morphology can be associated with a specific kind of dislocation or disclination defect (Fig. 4). Two defects can even combine to lead to more spohisticated morphologies (bagel, twisted toroid…).

Biomorph shapes have been mainly obtained with silica-based materials. Silica is a well known glass former that leads to amorphous solids. Curved surfaces are then much easier to form in the absence of crystallographic constraints. This is no longer the case when crystals are used as precursors. Flat surfaces corresponding to the crystallographic planes, with definite angles, are then usually observed. Curved surfaces could anyway be obtained via the controlled self-assembly of nanocrystals, leading to ordered superstructures called “mesocrystals” [8] and [9].

These mesocrystals somehow mimic the formation of biostructures found in the hammer-shaped building units of coccoliths or the skeletal plates of sea urchins.

Mesocrystals are formed via a bottom-up process that couples together chemical synthesis and self-assembly in order to produce complex superstructures. Syntheses are performed in the presence of organic additives that limit crystal growth and control their aggregation. Self-organization leads to single-crystalline structures with included organic additives as defects. Mesocrystals could be considered as a special case of colloidal crystals in which building units are anistropic nanocrystals rather than spherical colloidal particles. They could be described as “crystals of crystals” in which the building units are nanocrystals rather than ions or molecules.

The formation of mesocrystals is a two-step reaction in which organic additives play a double role (Fig. 5): •

in the first step, organic additives bind to some crystalline faces. Crystal growth is then limited leading to the formation of nanocrystals stabilized by the organic molecules adsorbed at their surface;

The synthesis of BaSO₄ (or CrO₄) mesocrystals provides a nice example of such a process. Linear chains of prismatic nanoparticles have been synthesized from a mixture of baryum sulfosuccinate reverse micelles and sodium sulfosuccinate microemulsions containing encapsulated sulfate or chromate anions. Slow exchange between these two components leads to the formation of BaSO₄ or BaCrO₄ nanoparticles [21]. The anionic groups (SO₄)^2– or (CrO₄)^2– preferentially bind to some faces of the crystals leading to the formation of prismatic nanoparticles. Interactions between organic chains of the surfactants then lead to the directed aggregation along the axis perpendicular to the larger faces in order to maximize the hydrophobic interactions. Nanocrystals within a chain are separated by a regular spacing of 2 nm corresponding to an interdigitated bilayer of surfactant molecules (Fig. 6).

Double hydrophilic block copolymers (DHBC) have been recently introduced to control the formation of mesocrystals [6] and [22]. They are made of two hydrophilic blocks. One is strongly interacting with the inorganic particles while the other one does not. In the PEG-b-PMAA block copolymer for instance, the carboxylic groups of the PolyMethAcrylic Acid (PMAA) strongly interacts with the inorganic particles while Polythylene Glycol (PEG) only provides solubility.

Complex morphologies have been obtained via the precipitation of BaSO₄ (or BaCrO₄) in the presence of the PEG-b-PMAA–PO₃H₂ block copolymer (Fig. 7) [33].

The complexing power of DHBC depends on the nature of the functional groups (–COOH, –PO₃H₂, –SO₃H) and their protonation state, i.e. on pH. A large variety of forms can be obtained [7] and [8]. Bundles of coiled single-crystalline BaSO₄ nanofilamnents are obtained in the presence of poly(ethylene oxide)-block-poly(methacrylic acid) (PEO-b-PMAA–PO₃H₂) [29] and [30].

Only a few examples have been described in this article. They show that chemical morphogenesis can lead to sophisticated structures. Preparing man-made materials with life-like forms becomes nowadays quite fashionable [23]. However, this is not the only aim of these research works. Shape is also very important for materials scientists. The properties of a given material often depend on its shape. Glass windows or fibres for instance are used for their transparency. Glass wool is used for its insulating properties and glass bottles as containers. Moreover, the development of nanotechnologies requires the synthesis of nanostructured materials. In this field, bio-materials provide a nice guide for the synthesis of bio-inspired materials.